The present invention relates to a process for the gasification of carboniferous material in solid, pulverulent or even lump form; the process has the feature that there is provided a fluidized bed and dust gasification, possibly also fixed bed gasification, taking place jointly in a reaction chamber consisting of one or several stages. The solids separated from the produced gas are returned to said reaction chamber.
Processes applying the three gas/solid reactions in a reaction chamber are known. The fraction of finer solid particles, which is carried along by the product gas from the fluidized bed, is separated in a cyclone which is arranged above the fluidized bed in the reaction chamber. The solid matter separated in the cyclone falls directly to a gasification equipment which is connected to the lower exit of the cyclone and which consists of one gasification burner as well as of one gasification chamber. The dust gasification products (gas, solids and molten slag) leave the gasification chamber and are blown from above onto the fluidized bed. In this way they transfer part of their sensible heat to the fluidized bed. The molten slag shall be consolidated and shall be discharged through the fluidized bed into the fixed bed arranged below.
When the dust gasification products are being blown onto the fluidized bed, that part of the sensible heat of the dust gasification products which is used for heating the fluidized bed is the smaller the lower the depth by which the gas jet penetrates into the fluidized bed. As a result the dust gasification gas may not be cooled down sufficiently and the mixed temperature of dust gasification gas and fluidized bed gasification gas on the way to the cyclone will be higher than the one obtained by complete heat exchange within the fluidized bed. This creates the danger that ash particles, especially fine fractions, will remain in the fusion zone and will stick to the walls of the reaction chamber on the way to the cyclone and in the cyclone itself.
Under the prior art fluidized bed gasification process, the separation of gas and of solids discharged from the fluidized bed takes place in a cyclone which is accommodated either in the reaction chamber and from which the solids are returned to the fluidized bed through legs or outside of the reaction chamber and from which the solids are likewise returned to the fluidized bed.
But because of their fineness, these solids returned will be discharged again from the fluidized bed very quickly so that only part of the pure substance contained in them can be converted there. Under certain circumstances the circulation of this solid fraction may rise to considerable values and may cause enrichment of the ash in the fluidized bed. In actual practice only part success is achieved in the attempt made to obviate this disadvantage by arranging a very voluminous so-called contact chamber above the fluidized bed; additionally, oxygen for postgasification is introduced into such a contact chamber.
Combining cyclone, gasification burner and gasification chamber in one unit under a supplementary embodiment involves the decisive disadvantage from the viewpoint of the user that maintenance of the sensitive dust gasification burner is not possible or is possible only at the expense of considerable downtimes, because cyclone, dust gasification burner and gasification chamber are arranged as one cooled unit in the rection chamber itself. This greatly restricts the availability of an operating plant.
A further considerable disadvantage is to be seen in the circumstance that this dust gasification equipment must be operated under blind conditions, because it is not possible to detect if any solid matter at all or how much solid matter passes to the gasification burner through the four exits of the cyclone. A correct relationship of the gasification agent, e.g. oxygen, can therefore not be established. For safety reasons, it is necessary to ensure that hydrogen or cleaned gas produced in the plant itself is supplied to the dust gasification in a quantity allowing the oxygen to be converted completely even in the event of the solids supply failing. But hydrogen or cleaned product gas is the most expensive energy carrier used in the process.
Both are burnt to water vapor or water vapor and carbon dioxide, respectively, with the generation of a very high temperature and shall again convert, with the pure substance of the solids, to form hydrogen and carbon monoxide. It is an established fact that such processes do not take place completely, the less so the poorer the solid/gasification agent mixture and the shorter the time available at a given reaction temperature.
Under the prior art these mix ratios are not optimum, because the solids separate from the gasification agent under the effect of the centrifugal force.
Moreover, a gasification time of only 0.05 seconds is available in the prior art dust gasification equipment. This time only suffices for converting a very small part of the solid matter, because at a dust grain size of 0.1 mm at least 0.2 to 0.3 seconds are required for complete conversion at reaction temperatures in excess of 1,600.degree. C.
It was also left out of consideration that under the prior art dust gasification processes, grain sizes of 0.1 mm, i.e. mean grain sizes of around 0.025 mm, are used, while the dust is discharged from the fluidized bed at grain sizes of up to 1 mm. The solids returned have, consequently, a mean grain size which, by at least one decimal power, is higher than upon dust gasification. This is the reason why at equal reaction temperatures and times also lesser pure substance of the returned solid matter is converted.
Under the prior art process, also the ash shall be fused completely, shall be granulated in the fluidized bed below, and shall be discharged from such bed. But at a reaction temperature of 1,600.degree. C. and an ash fusion point of, say, 1,300.degree. C., an ash grain of 0.1 mm in diameter requires already a fusion time which is close to 0.3 seconds.
This means that in the prior dust gasifier part, only a small fraction of the pure substance is converted and only a small part of the ash is fused.
In this way the quantity of the circulating and permanently returning solid matter is increased so that under certain circumstances the amount of solid matter introduced through the gasification burner is exactly the same as the one or is larger than the one admitted to the fluidized bed and fixed bed zones together. But this results also in a high oxygen and gas demand, because the solid matter returned several times must also be heated several times from, say, 1,000.degree. C. to 1,600.degree. C. Furthermore, it is then no longer possible to coordinate the heat balance between the fluidized bed and the dust gasification products, because the fraction of the dust gasification products is too high.